2004 Progress Report: Models and Measurements for Investigating Atmospheric Transport and Photochemistry of HgEPA Grant Number: R829799
Title: Models and Measurements for Investigating Atmospheric Transport and Photochemistry of Hg
Investigators: Keeler, Gerald J. , Sillman, Sanford
Institution: University of Michigan
EPA Project Officer: Chung, Serena
Project Period: November 1, 2002 through October 31, 2005 (Extended to October 31, 2006)
Project Period Covered by this Report: November 1, 2003 through October 31, 2004
Project Amount: $899,597
RFA: Mercury: Transport, Transportation, and Fate in the Atmosphere (2001) RFA Text | Recipients Lists
Research Category: Mercury , Air Quality and Air Toxics , Safer Chemicals , Air
The objective of the research project is to develop a 3-dimensional Eulerian model for transport and photochemistry of mercury, including fully integrated gas-phase, aqueous, and aerosol chemistry. An important goal is to develop detailed comparisons between model results and measurements from two field campaigns: a field campaign in south Florida involving aircraft measurements of mercury and an ongoing campaign being performed in the Great Lakes region.
The project also seeks to use model results to investigate several research issues related to mercury: (1) the impact of local emissions relative to transport from distant sources; (2) the impact of photochemical processes; and (3) the relative importance of cloud chemistry on the transformation, transport, and deposition of mercury. A major objective will be to identify measurements that might provide evidence relevant to these issues.
The accomplishments during Year 1 and Year 2 of the project are described in this section. We implemented a combined numerical solution for gas-phase and aqueous-phase chemistry, including gas-aqueous interactions on short (<1 second) time scales, in the Community Model for Air Quality (CMAQ). We added aqueous reactions for sulfates, nitrates, ozone, hydrogen peroxide, OH and related radicals, chlorine, and mercury, along with reactions from a relatively complete mechanism for gas-phase chemistry. This also included bromine and chlorine chemical reactions in the full gas and aqueous phase chemical scheme. We applied the mercury model to a test case simulation for a model domain that included the eastern half of the United States and the western half of the Atlantic Ocean (from 55 to 105° W longitude).
As part of the project, a modified version of CMAQ has been developed that includes a numerical solution for combined aqueous and gas-phase chemistry. This combined solution accounts for the rapid exchange between gas-phase and aqueous-phase that takes place within clouds, and has been tested in an inter-comparison of solutions for gas- and aqueous-phase chemistry (Barth, et al., 2003).
During the first 2 years of the project, a modified version of CMAQ was developed that included emissions, photochemistry, transport, and deposition of elemental and reactive gaseous mercury (RGM). The modified CMAQ included an integrated solution for combined gas-phase and aqueous chemistry, in place of the original CMAQ procedures that solve for gas-phase and aqueous chemistry separately. The integrated solver accounts for interactions between gas-phase and aqueous chemistry on short time scales (<1 second) and also allows for a much more detailed representation of aqueous chemistry. Emissions for mercury were added, derived from the U.S. Environmental Protection Agency’s 1999 Hazardous Air Pollutants (HAP) Inventory version 3. During Year 2, the model gas and aqueous chemistry was expanded to include chlorine and bromine chemistry, in addition to the gas and aqueous chemistry of mercury, sulfur, and ozone and related species.
- Figure 1. Concentrations of Hg(II) (pg m-3) in the Model at 3000 m Altitude for the Full Model Horizontal Domain. Results are for 5:00 p.m., June 14, 2000. The colors represent the following concentration intervals: grey, 0-40 pg m-3; dark blue, 40-80 pg m-3; light blue, 80-120 pg m-3; green, 120-160 pg m-3; light green, 160-200 pg m-3; orange, 200-240 pg m-3; and red, 240-280 pg m-3.
During Year 2, the model was used to simulate conditions for 11 days in June, 2000 for a model domain that included the eastern half of the United States and the western half of the Atlantic Ocean (from 55 to 105° W longitude). The goal of the simulation was to assess the photochemical formation of reactive mercury at the regional scale, including photochemical evolution over 5 days or longer. The selected time period also coincided with a series of aircraft measurements in south Florida.
The model predicted that high ambient concentrations of RGM (up to 200 pg m-3) can be formed through photochemical conversion of Hg0 to RGM. The high RGM was predicted to be episodic in nature, with the highest RGM coinciding with extended cloud-free periods. Elevated RGM is not limited to source regions (such as the Eastern United States) and can form over the Atlantic Ocean as well (see Figure 1). A comparison with aircraft measurements in south Florida (Figure 2) shows that the model reproduces several observed features. RGM increases with elevation (up to 3000 m) in both the measurements and the model. Measured RGM shows intermittent high RGM with concentrations up to 250 pg m-3. Although the highest measured RGM was twice as high as the model value in south Florida, the model did predict up to 240 pg m-3 in nearby regions of the Atlantic Ocean.
Figure 2. Measured Hg(II) (pg m-3) Versus Altitude (km) From Aircraft Measurements Over South Florida and the Adjacent Atlantic Ocean During June 2000 (points). The line represents model Hg(II) versus altitude, based on an average of model results during the afternoon on the five days (June 9, 12, 14, 25, and 26) that coincide with measurements.
Tests have been completed for model sensitivity to direct emissions, background Hg0, and chemistry, including tests for the impact of a specific reaction (the aqueous reaction of Hg(II) with HO2) that recently was found to be uncertain.
It was found that ozone formation during pollution events can significantly affect ambient RGM. The model predicts a 50 percent reduction in RGM if emission of ozone precursor emissions was eliminated. The model predicted that RGM should show a significant positive correlation with ozone in cases where RGM is enhanced as a result of ozone formation. This correlation should provide a way to test for the validity of the model prediction concerning the impact of ozone on RGM.
The following activities are planned for 2005-2006:
- Complete the interpretation of results from the regional model described above, focusing on the predicted wet deposition of mercury.
- Develop an extended comparison of model results with measurements, focusing on predicted model correlations (RGM vs. Hg0, RGM vs. ozone) that are closely linked with model predictions concerning the sources of RGM (direct emissions, photochemistry, etc.) and might therefore be used to test the validity of model processes.
- Develop model scenarios using fine grid resolution to investigate processes in south Florida, including the impact of power plants, which are not well represented in the current regional horizontal grid.
- Develop model scenarios for comparison with measurements in the Great Lakes region.
- Submit current results for publication in peer-reviewed literature.
Further integration between the gas-aqueous and aerosol components of the CMAQ model also is needed, but it will not likely be completed during the next year due to the complexity of this task.
Barth M, Sillman S, Hudman R, Jacobson MZ, Kim C-H, Monod A, Liang J. Summary of the cloud chemistry modeling intercomparison: photochemical box model calculation. Journal of Geophysical Research 2003;108(D7), 4214, doi:10.1029/2002JD002673.